Cryogenic vacuum pumps, or cryopumps, currently available generally follow a common design concept. A low temperature array, usually operating in the range of 4 to 25K, is the primary pumping surface. This surface is surrounded by a higher temperature radiation shield, usually operated in the temperature range of 60 to 130K, which provides radiation shielding to the lower temperature array. The radiation shield generally comprises a housing which is closed except at a frontal array positioned between the primary pumping surface and a work chamber to be evacuated.
In operation, high boiling point gases such as water vapor are condensed on the frontal array. Lower boiling point gases pass through that array and into the volume within the radiation shield and condense on the lower temperature array. A surface coated with an adsorbent such as charcoal or a molecular sieve operating at or below the temperature of the colder array may also be provided in this volume to remove the very low boiling point gases such as hydrogen. With the gases thus condensed and/or adsorbed onto the pumping surfaces, only a vacuum remains in the work chamber.
In systems cooled by closed cycle coolers, the cooler is typically a two-stage refrigerator having a cold finger which extends through the rear or side of the radiation shield. High pressure helium refrigerant is generally delivered to the cryocooler through high pressure lines from a compressor assembly. Electrical power to a displacer drive motor in the cooler is usually also delivered through the compressor.
The cold end of the second, coldest stage of the cryocooler is at the tip of the cold finger. The primary pumping surface, or cryopanel, is connected to a heat sink at the coldest end of the second stage of the cold finger. This cryopanel may be a simple metal plate or cup or an array of metal baffles arranged around and connected to the second-stage heat sink. This second-stage cryopanel also supports the low temperature adsorbent.
The radiation shield is connected to a heat sink, or heat station, at the coldest end of the first stage of the refrigerator. The shield surrounds the second-stage cryopanel in such a way as to protect it from radiant heat. The frontal array is cooled by the first-stage heat sink through the side shield or, as disclosed in U.S. Pat. No. 4,356,701, through thermal struts.
After several days or weeks of use, the gases which have condensed onto the cryopanels, and in particular the gases which are adsorbed, begin to saturate the cryopump. A regeneration procedure must then be followed to warm the cryopump and thus release the gases and remove the gases from the system. As the gases evaporate, the pressure in the cryopump increases, and the gases are exhausted through a relief valve. During regeneration, the cryopump is often purged with warm nitrogen gas. The nitrogen gas hastens warming of the cryopanels and also serves to flush water and other vapors from the cryopump. By directing the nitrogen into the system close to the second-stage array, the nitrogen gas which flows outward to the exhaust port minimizes the movement of water vapor from the first array back to the second-stage array. Nitrogen is the usual purge gas because it is inert and is available free of water vapor. It is usually delivered from a nitrogen storage bottle through a fluid line and a purge valve coupled to the cryopump.
After the cryopump is purged, it must be rough pumped to produce a vacuum about the cryopumping surfaces and cold finger to reduce heat transfer by gas conduction and thus enable the cryocooler to cool to normal operating temperatures. The rough pump is generally a mechanical pump coupled through a fluid line to a roughing valve mounted to the cryopump.
Control of the regeneration process is facilitated by temperature gauges coupled to the cold finger heat stations. Thermocouple pressure gauges have also been used with cryopumps but have generally not been recommended because of a potential of igniting gases released in the cryopump by a spark from the current-carrying thermocouple. The temperature and/or pressure sensors mounted to the pump are coupled through electrical leads to temperature and/or pressure indicators.
Although regeneration may be controlled by manually turning the cryocooler off and on and manually controlling the purge and roughing valves, a separate regeneration controller is used in more sophisticated systems. Leads from the controller are coupled to each of the sensors, the cryocooler motor and the valves to be actuated.
A cryopump comprises a cryogenic refrigerator, a gas condensing cryopanel cooled by the refrigerator, at least one temperature sensor coupled to the cryopanel and an electrically actuated valve, such as roughing valve, adapted to remove gases from the cryopump. In accordance with the present invention, an electronic processor is an integral part of the cryopump assembly and is coupled to the sensor to provide a temperature indication, to the valve to control opening and closing of the valve and to the refrigerator to control operation thereof.
Preferably, the electronic processor is mounted in a housing of a module which is adapted to be removably coupled to the cryopump. A control connector on the module is adapted to couple the electronics, to a refrigerator motor, to the temperature sensor in the cryopump and to the valve. A power connector is adapted to connect the electronics to a power supply. The electronic module may store system parameters such as temperature, pressure, regeneration times and the like. It preferably includes a nonvolatile random access memory so that the parameters are retained even with loss of power or removal of the module from the cryopump. The module may be programmed to control a regeneration sequence. Preferably, a heater is mounted integrally with the cryopumping arrays, and a purge valve is mounted to the system. The electronic module controls those devices as well.
Preferably, the electronic module has the control connectors and power connectors at opposite ends thereof, and it is adapted to slide into a housing fixed to the cryopump. The module is locked in place such that it cannot be removed so long as a power lead is coupled to the connector. A keyboard and display may be pivotally mounted at the end of the fixed housing opposite to the end in which the module is inserted and thus opposite to the power connector. Preferably, the display is reversible to allow for both upright and inverted orientations of the cryopump.
The processor may be programmed to provide a number of enhancements to the system. For example, after a power failure, the system may check to determine whether the sensed temperature is sufficiently low to permit a successful restart of the cryopump and, if so, to start the refrigerator motor. If not, the processor may initiate a regeneration cycle. The system may automatically zero a thermocouple pressure gauge after each regeneration. Regeneration may be improved by directly heating the array with the heaters throughout the rough pumping procedure. To hasten the regeneration process, the rate of pressure drop may be monitored, and a portion of the regeneration procedure may be repeated where the rate falls below a predetermined setpoint before the pressure reaches a sufficiently low level. Warnings may be provided to a user before the user is allowed to complete a task, such as opening of a valve, in a situation which might contaminate the system or cause other problems. Temperature sensing diodes may be used with high precision by individually calibrating each diode and storing calibration data with the processor.
Access through the keyboard may be limited until a predetermined password has been input. For example, use of the keyboard and display may be limited to monitoring of system parameters, and control of the system may be prohibited without the password. Within a routine which is always protected by the password, an operator may determine whether other functions are also to be protected.
A password override may be obtained from a trusted source who has access to an override encryption algorithm. The algorithm is based on a varying parameter of the system which is available to any user. The electronic processor includes means for determining the proper override password through the same encryption algorithm. The parameter of the system may, for example, be the time of operation of the system. As a result, an operator may be allowed to override the password on select occasions without having the ability to override in the future.
Individual and local electronic control of each cryopump has many advantages over strictly central and remote control. Although the present system has the advantage of being open to control and monitoring from a remote central station, control of any pump is not dependent on that central station. Therefore, but for a power outage, it is much less likely that all pumps in a system will be down simultaneously. The local storage of data such as calibration data and data histories are readily retained in the local memory without requiring any access to the central station. Thus, for example, in servicing a cryopump by replacing a module, the service person need not input any new data into the central computer because all necessary information is retained and set at the pump itself. Also, in servicing a pump, it is much more convenient to the service person to have full control of the pump when he is at the pump itself rather than having to seek control through a remote computer. The local full control of the cryopump facilitates enhancements to individual pumps because there is no burden on the central computer. As a result, many procedural improvements which provide faster, more thorough regeneration are more likely to be implemented. The removable module greatly facilitates servicing of the unit, and the battery-backed memory allows such servicing without loss of data. The module also facilitates upgrading of any individual pump.